This invention provides a clutch for a positive feed automatic drill with reversible motor rapid advance that slips when the drill spindle encounters excess torque in reverse to prevent damage to the drill, the drill bit or to the workpiece. The invention provides a clutch to be used in combination with a positive feed drill of the type disclosed and claimed in U.S. Pat. No. 4,591,299 and assigned to the same assignee as the present application. The disclosure of U.S. Pat. No. 4,591,299 is hereby incorporated by reference.
The invention disclosed in U.S. Pat. No. 4,591,299 provides a rapid feed apparatus to rapidly advance a drill bit to the surface of a workpiece. This is accomplished by running the motor in reverse with the feed gear locked or yieldably restrained from rotating. This causes the spindle to rapidly advance towards the workpiece with the bit rotating in a reverse direction. A yieldable or positive stop device located in the housing engages and prevents rotation of the feed gear when the feed gear is moved out of driving engagement with the main drive gear. The stop apparatus is yieldable when a predetermined torque is placed on the feed gear by the spindle feed gear during retraction or rapid feed movement so that the spindle feed gear rotates at the same speed as the spindle, thereby stopping the retraction or rapid feed movement of the spindle.
With a positive stop or even with a yieldable stop included in the drill, problems arise with excessive torque or torque overload during the rapid advance of the drill bit towards a workpiece. For example, the bit could wedge or lodge into an existing hole in the workpiece. The chuck or the depth stop or any of the spindle mounted accessories could strike an immovable part. Additionally, as the cutter encounters the workpiece in a reverse rotation it could bind causing a torque overload. If any of these should happen, or if for any reason the spindle is prevented from rotating, several things may occur: the drill head can unscrew from the tool nose, the drill may unscrew from its mounting, the drill bit may be damaged, or the gears may experience excessive torque causing a single or multiple failure of gear teeth.
To avoid these undesirable results, the present invention provides a clutch that has been devised to be used in conjunction with a positive feed drill and a reversible motor rapid advance system. The clutch is designed so that it is positive drive in a forward rotation but acts as a torque-limiting device in reverse (the rapid spindle advance mode). Therefore, it prevents damage to the drill, drill bit or to the workpiece, and it prevents unscrewing of the drill head or bit.